Here it’s important to pay attention to the way Tumblr actually works — or more precisely, the two ways it works.

Tumblr offers tools to make simple blog pages, which anyone with a Web browser can see. So you don’t have to sign up for Tumblr to check out We Want Porn, but comScore will count you as one of the service’s 117 million monthly users.

Tumblr’s core users, though, log in to the service, and subscribe to different Tumblogs, which they view on a “dashboard” — the equivalent of Twitter and Facebook’s newsfeeds.

Not coincidentally, these are also the only people Tumblr is showing ads to, either via “radar” ads that promote Tumblr pages alongside users’ dashboards, or “spotlight” ads that promote Tumblr pages in a directory of suggested accounts.

To spell that out: Tumblr’s advertisers don’t have to worry about their stuff showing up on blogs like We Want Porn. At worst, it’s possible that they’ll end up advertising to a user whose dashboard includes posts from We Want Porn. But in general, they ought to be pretty well insulated from that stuff.

By the same token, if Yahoo wanted to, it could end up scrubbing Tumblr of porn, and losing a lot of users and views — but it probably wouldn’t lose much in the way of monetizable users. Unless it turns out that the majority of Tumblr’s core users have signed on exclusively to use porn.

So: Problem? Sure. But it doesn’t look like a costly one.

Update: Now that the deal is done, a Tumblr backer has piped up to offer a more concise version of my argument: “Non-story. Tumblr is the Internet. It’s a dashboard follower model, opt-in.” That would have saved me a bunch of typing!

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